Washington: With Donald Trump emerging as the undisputed frontrunner in the presidential nomination race, the Republican establishment mounted a concerted attack on the brash billionaire on and off stage only to have the tables turned on them.

The Republican debate Thursday night began with the real estate mogul's closest rivals Texas senator Ted Cruz and Marco Rubio taking to the attack from where the party's 2012 nominee Mitt Romney had left off hours earlier.

But in the end, all three of Trump's remaining rivals - Cruz, Rubio and Ohio governor John Kasich -- pledged on the debate stage to support the party's eventual nominee -- even if that ends up being Trump.

Trump -- who has repeatedly threatened the possibility of launching a third party bid should he feel that the party is treating him unfairly -- also said he would back whoever the party crowns. "The answer is, yes, I will."

The debate began with the anchors bringing up Romney's sharp condemnation of Trump earlier Thursday calling him "a phony, a fraud" in a speech in Salt Lake City.

"His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers. He gets a free ride to the White House, and all we get is a lousy hat," Romney said.

But Trump hit back during the debate and before, calling Romney an "embarrassment" to the Republican Party.

"He was a failed candidate, he should have beaten President Obama very easy. He failed miserably and it was an embarrassment to everybody," Trump said. "I guess obviously he wants to be relevant, he wants to be back in the game."

During a rally in Maine earlier Thursday, Trump blasted Romney for "begging" for his endorsement.

"I don't know what happened to him," Trump said. "You can see how loyal he is. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees."

Trump opened the debate with his devastating response to Rubio joking about the size of Trump's hands and saying "you know what they say about men with small hands."

On the debate stage, Trump stretched his hands out for the audience to see - then insisted that the suggestion that "something else must be small" was false.

"I guarantee you there's no problem," Trump said to howls from the audience at the Fox debate.

Trump fielded attacks from Cruz and Rubio throughout the night.

Rubio, who is banking on winning his home state of Florida on March 15, went after the Republican frontrunner for selling products made in China and Mexico.

Trump shot back: "This little guy has lied so much about my record. He has lied so much about my record."

Cruz, as he has done for weeks, also attacked Trump for having supported and donated money to Democrats in the past.